1. Field of the Invention
The present invention relates to a semiconductor device including a coil.
2. Description of the Related Art
There has been a semiconductor device including a coil defined by a metal wire layer formed in a spiral, so as to obtain the desired inductance. FIG. 6 shows an example of an inductor (coil) included in such a semiconductor device. In this inductor 101, the occupying area increases two-dimensionally if the number of turns of one layer of the metal wire layer 111 increases, so the occupying area and the length of the metal wire required for a unit number of turns increases according to the number of turns. Therefore, in order to achieve a large inductance value L, it is a matter of course that the occupying area of the inductor 101 increases, and the resistance value R of the metal wire constituting the inductor 101 per unit number of turns also increases, and the value Q, which indicates the sharpness of resonance, given by Q=ω0×L/R drops (ω0 is the resonance angular frequency). In FIG. 6, 118 is a terminal formed on another metal wire layer, and 113 is a connection hole for electrically connecting one metal wire layer 111 and the terminal 118.
In an attempt to solve the above-described problems, a semiconductor device including an inductor, which does not increase the occupying area and the length of the metal wire per unit number of turns even if the number of turns is increased, by forming the coil three-dimensionally, has been proposed (e.g. Japanese Patent Application Laid-Open No. H6-21347, and Japanese Patent Application Laid-Open No. H7-273292). FIG. 7 shows an example of this three-dimensional inductor. In this inductor 201, a plurality of pieces 211 formed on the bottom metal wire layer and a plurality of pieces 212 formed on the top metal wire layer are disposed in parallel respectively, sandwiching an inter-layer insulation film, which is not illustrated, connecting these plurality of pieces 211 and 212, so as to define a serial connection, via the connection holes 213, so as to form a three-dimensional spiral. In this inductor 201, the number of turns per area is high, and the length of the metal wire to receive a predetermined inductance value can be small, so the above mentioned value Q can be increased. In FIG. 7, 219 is a terminal at one side connected to the piece 211 to be the start edge, and 218 is a terminal at the other side connected to the piece 211 to be the end edge.
In order to further increase the inductance value, as shown in FIG. 8, an inductor 202, having a structure including a ferro-magnetic core 215 which is formed by an insulation film sandwiching an iron conductor layer or an oxide film containing an iron substance, for example, between the pieces 211 and 212 of the top and bottom metal wire layers, has been proposed (e.g. above-mentioned Japanese Patent Application Laid-Open No. H6-21347, and Japanese Patent Application Laid-Open No. H7-273292). This inductor 202 can increase the magnetic flux density passing through the ferro-magnetic core 215, so the inductance value can be further increased.
The three-dimensional inductor 202 including the ferro-magnetic core 215 shown in FIG. 8, however, requires special steps of forming the ferro-magnetic core 215 between the pieces 211 and the pieces 212 of the top and bottom metal wire layers, which causes increased costs and low yield.
The inductor 201, not having the ferro-magnetic core 215, on the other hand, can increase the inductance value by increasing the occupying area thereof, but if the semiconductor device has a practically appropriate chip size, the inductance value to be implemented is smaller than the inductor 202 with the ferro-magnetic core.